There have been continuous efforts over the years to design portable electric appliances so that the user would be free of risk of accidental injury or death as a consequence of electric failures or current leakage conditions in such appliances. With the advent of plastic materials which are good electrical insulators and are strong from a structural standpoint, the goal of separating and insulating the user from the electrically energized portions of electrical appliances has been made much easier to achieve.
Most of the portable hand-held appliances which present the greatest risk to the consumer are provided with plastic housings which totally enclose any energized electric parts of the appliance. As long as the housing itself remains intact, there is little risk of the user contacting an electrically energized portion of the appliance even if there is an internal failure which might take the form of a lead wire shorted to the motor frame or some other similar fault or breakage. In spite of these improvements in the basic construction of portable appliances however, there is still one area in which studies show that there have been serious injuries and death to the user.
Studies indicate that the most serious risks relate to the use of portable electric appliances in the bathroom where there is water present and the user is often well grounded to receive the maximum injury from any leakage current from an appliance. The appliances typically involved in injuries in the bathroom are hair dryers hair curlers, styling wands, massagers and any other grooming appliance which might be plugged in while being handled by the user in the bathroom. Problems have typically arisen when a user will accidentally drop the hair dryer into a bathtub or sink and then seek to retrieve it without disconnecting the plug from the power outlet There have also been many instances in which children have been injured when playing with hair dryers in the bathtub. There are also serious risks involved in the use of many portable electric tools such as drills, saws, sanders, etc. since they are often used outside and in wet environments where electrical faults can present serious hazards. In such instances, the user is well grounded and offers a low impedance path for the leakage current and, as a consequence, fatalities have occurred. It should be appreciated that the current encountered by a user in such a situation is not large, usually less than one ampere with a 120 volt power supply. However, the heart may be seriously affected by such alternating currents if not interrupted promptly.
There have been many studies made to determine the nature of the dangers involved in immersing an electric appliance in water and studies of the types of injuries resulting therefrom. The nature of the leakage current produced in such an immersion situation varies considerably. Factors involved in the level of the leakage current through a person grasping an immersed appliance would be the magnitude of the supply voltage, the resistance of the water, the size and resistance of the person involved, the availability of a ground, and the current path as determined by the physical disposition of the appliance as compared to the location of the ground.
The resistance of normal tap water can vary anywhere from 1,000 to 100,000 ohm centimeters/square depending on the impurities found in any geographical area. The resistance of water in ohm centimeters/square can be calculated from available data on the average total dissolved solids in milligrams/liter. However, with an average resistance of 20,000 ohms at 120 volts, a leakage current of 35 milliamps might be considered average. Such a current is much too small to cause any tissue damage to a human being but, as was indicated above, is sufficient to incapacitate a person and after a period of time accelerate the heartbeat and cause death. The basic objective in any protective circuit is to respond in a reasonably short time to leakage currents which are sufficiently large to present risk of injury to humans.
There have been many attempts in the past to provide safety or protective circuits which would lessen or eliminate the hazards described above. In most cases, such attempts have been less than perfect in eliminating the risks and have been characterized by being complicated and expensive to construct. The typical portable electric hair dryer often sells for less than $10.00 while many of the proposed safety circuits would cost more than the hair dryer itself.
One of the common types of protective circuits is known as a ground fault interrupter circuit. Such a circuit is designed to sense and respond to the power which is actually passing from the controlled or regulated appliance to the ground. While this provides a protective approach to sensing and responding to the kind of leakage currents which are dangerous to humans, it requires that the appliance itself be well grounded. The sensing means would frequently include means for sensing the power which is lost or otherwise not delivered to the load. Such circuits are characterized by being complex, expensive and unsuitable for use with an inexpensive appliance which is normally used in a situation where it is ungrounded.
One early example of a prior art protective circuit is shown in the Hurtle U.S. Pat. No. 3,493,815 which is designed for use with a grounded two wire power system or a three phase system. The Hurtle patent employs a three wire conductor which may be connected between the appliance and a safety device interposed between the appliance and the power supply. In addition to the normal two power conductors, a third wire connects the motor frame to a triac which is triggered by a current flow from the load to short circuit the load through a line fuse to interrupt the circuit. In one embodiment of the Hurtle patent, there is a single line fuse, but in another there are fuses in both lines and two triacs to sense the current flow to the frame of the appliance from either side of the line. Since all of the embodiments of Hurtle designed for use with a two wire power supply have a grounded line, it should be recognized that a commercially acceptable protective circuit cannot depend on having a grounded line available in the normal domestic situation. Consequently, there are instances in which the Hurtle patent would not provide adequate protection. One such instance is in a situation in which one of the line fuses is blown as a consequence of the appliance being immersed in water. If the user were to then remove the appliance from the water and plug it in with a reversed polarity, the protective device of the Hurtle patent would be ineffective in blowing the second fuse and would leave the appliance still dangerous to the life of the user. In this particular instance, the appliance might still have sufficient water within the housing to provide enough leakage current to seriously injure or kill the user.
Other examples of prior art devices intended to protect the user from such injury are Gross U.S. Pat. No. 4,031,431 and Bienwald et al U.S. Pat. No. 4,412,193. The safety circuits disclosed in both of these patents are complicated and would be expensive to add to a portable home hair dryer.